The samples used to make these patterned devices are again fabricated by Dr. R. P. Vasquez at the Jet Propulsion Laboratory. These samples are similar to the ones made using the shadow masking, described in Sec. 3.1, except that the samples are larger squares
(1 1 cm) and that an additional layer of gold has been pre-evaporated in situ, allowing
for contacts to be made to the devices. This additional Au layer also serves the purpose of a protective layer to the YBCO …lm underneath, as the procedures of lithography require sample to come in contact with water and other organic solvents, which may prove to be detrimental to the survival of the superconductor otherwise. The …rst set of F-I-S devices is made on YBCO/STO/LSMO of thicknesses 100 nm/2 nm/100 nm, and the second set
on similar sample of thicknesses 100 nm/3.5 nm/150 nm. Partner samples with LNO
underlayer are processed as well as controls.
The procedures for the microfabrication can be divided into four major stages to achieve these features: CMR mesa, YBCO bridge, Au contacts, and contact wires. Each
Figure 8.1: First stage in the microfabrication to obtain the large CMR mesa.
stage includes similar series of steps. The details on photolithography (Sec. 8.1) and thin …lm processes (Chapter 7) will be abbreviated to avoid repetition. Relevant comments will be noted.
First, the photolithography procedures discussed in the previous section are fol- lowed to obtain the patterns for the large CMR mesa structures. The sample is then placed
in the ion etch chamber for a quick reactive O2 ion etch at low power (20%) in 40 mTorr
for 20 seconds. This serves to remove any residual layer of organic material in the exposed region in the patterning. To etch the CMR mesa, an ion beam at 500 V, 50 mA, with an
accelerator voltage at 800 V is applied. The milling pressure is 2 10 4 Torr. The ion
milling rate for the YBCO and CMR layers are comparable, with the CMR layer etching
nearly 10% slower. After the CMR mesa etching is complete, the photoresist is stripped
with acetone, etc., and another reactive O2 ion etch for 5 minutes follows to remove the
deposits and other adhesions on the sample surface from the photoresist and the organic
Figure 8.2: Second stage in the microfabrication to obtain the YBCO bridges.
measure an average height across the edges of the mesa.
The second stage involves patterning the narrow YBCO bridges. Here the pro-
cedure mimics those described for the CMR mesa. However, the e¤ect of edge beads
become a factor, so their removal need to be addressed. It is also necessary to align the
patterns properly relative to the existing CMR mesa. As one may expect, since the etch depth should extend only down to the CMR layer (as opposed to all the way to the LAO substrate), the etch time should be reduced, according to the calibrated rate. Again, the bridge height should be veri…ed using the pro…lometer.
The third stage of the fabrication process is to obtain the Au contacts. The
photolithography requires a rather di¢ cult alignment procedure as some of the contacts have dimensions of only a few microns. Successful patterns for contacts of these sizes require
very clean sample surface and good photomask-to-sample contact. Since the material to
be etched is the top Au layer of the sample, which has a much faster etch rate compared to perovskite materials, the ion beam voltage and current are decreased to 100 V and 10
Figure 8.3: Third stage in the microfabrication to obtain the Au contact pads and to add insulator to prevent shorting in the devices.
mA, respectively. Immediately after the ion etch, a layer of SiO or SiO2 insulator layer of
thickness200 300nm is deposited on the entire substrate surface. Therefore, upon removal
of the photoresist after the deposition step, only the previously protected Au contact areas will be exposed for the purpose of electrical contact, and everywhere else will be covered by this insulator layer.
The last stage is the patterning of the gold contact wires from the edge of the sample to the Au contacts on the devices. The lithography performed in this step can be the same process as in previous stages, or the negative-tone resist can be used, where the
Figure 8.4: Final stage in the microfabrication to obtain the contact wires.
image reversal is accomplished via a simple post-exposure bake process. The deposition of
gold is done in an evaporator for about3 4minutes for a thickness of 200nm. To assist
in the adhesion of Au to the SiO/SiO2insulator layer, a5nm primer layer of Ti is deposited
prior to the Au deposition. The two metal layer deposition can usually be evaporated in
the same evaporation chamber in one single pump down. Then the photoresist removal
and reactive O2 ion etch cleaning completes this last stage of processing. Fig. 8.5 is a
photographic image taken of the sample on the scale of nearly the entire die, with the devices located near the center of the square die. Fig. 8.6 shows the close-up of the F-I-S devices.
Figure 8.5: Image of one spin injection sample, which was fabricated at JPL. The mask used for patterning of this sample was designed by Dr. Je¤rey Barner and Dr. Alan Kleinsasser.